When a semiconductor device is manufactured, various heat processing apparatuses are used for subjecting a semiconductor wafer, which is an object to be processed, to processes such as an oxidation process, a diffusion process, and a CVD (Chemical Vapor Deposition) process. A general heat processing apparatus includes a heat processing furnace which is composed of: a processing vessel (reaction tube) capable of accommodating semiconductor wafers and heat-processing the same; and a heater (heating device) that is disposed to cover a circumference of the processing vessel, for heating wafers in the processing vessel. The heater has a cylindrical heat insulating member, and a heating resistor disposed on an inner circumferential surface of the heat insulating member via a supporting member.
In a heat processing apparatus capable of performing a batch process, for example, there is used, as the heating resistor, a helical heating element (also referred to as “heating wire” and “heating resistor”) arranged along an inner wall surface of the cylindrical heat insulating member. The heating element can heat an inside of the furnace to a high temperature such as about 800° C. to 1000° C. As the heat insulating member, there is used a member that is formed by burning a heat insulating material, such as ceramic fibers, into a cylindrical shape. The heat insulating member can reduce a heat quantity lost as radiant heat and conductive heat, so as to enhance efficiency in heating. As the supporting member, there is used a ceramic member, for example. The ceramic supporting member can support the heating element at predetermined pitches, while allowing heat expansion and heat shrinkage of the heating element.
In the above heat furnace, in order that the heating element can be thermally expanded and thermally shrunk, the heating element, which is helically formed, is supported such that a clearance is defined between the heating element and the heat insulating member. However, by using the heating element under a high temperature, the heating element undergoes a creep strain, and slowly increases in length over time. In addition, the heating element is thermally expanded during a heating operation. On the other hand, there is an apparatus that quickly cools the heating element by blowing air thereto so as to decrease a temperature. Due to the repeated rise and drop in temperature, the heating element is likely to be deformed. This may generate a short-circuit between adjacent parts of the deformed heating element, which may invite disconnection.
Particularly in a vertical-type heat processing furnace, the heating element is moved in the supporting member because of the repeated heat expansion and heat shrinkage caused by the rise and drop in temperature, and the heating element is moved downward little by little because of gravitation. Then, the moving amount is accumulated at a lowermost turn of the heating element. Namely, because of the accumulation of the movement of the heating element, a winding diameter of the lowermost turn is increased. When the heating element of the increased winding diameter reaches an inner surface of the heat insulating member and cannot be expanded outside any more, the heating element is then deformed in the up and down direction. Thus, there is a possibility that a short-circuit occurs between a part and another part adjacent thereto of the heating element, resulting in some disconnection.
In order to solve these problems, the following structure has been proposed. Namely, with a view to preventing such an accumulation to one side of the elongated heating element caused by creep and thermal expansion or the like, a fixing member of a rod-like shape is attached to an outside portion of the heating element by welding, and a distal end of the fixing member is buried to be fixed in a heat insulating member, so that the fixing member projects outward in a radial direction of the furnace (see, Patent Document 1).    [Patent Document 1] JP10-233277A    [Patent Document 2] JP2005-197074A
However, in the above structure in which the fixing member is merely joined to the outside portion of the heating element by welding, the joined portion is exposed to a high temperature. In addition, it can be considered that a stress tends to concentrate on the joined portion when the heating resistor is thermally expanded or thermally shrunk, which entails deterioration in durability (reduction in lifetime) of the heating element. Further, since the fixing member has a bar-like shape, the fixing member may easily drop out of the heat insulating member, whereby it is difficult to secure a sufficient holding force for the fixing member.
Moreover, when it is desired to quickly increase or decrease the temperature of a wafer, a large power has to be applied to a heating element during the quick temperature-increasing operation. However, a conventional, general heating element may not withstand the large load, and may be prone to be disconnected. For this reason, such a large power cannot be actually applied, and thus the quick temperature increase/decrease operation has been limited. Although the use of a heating element resistant to disconnection can overcome the difficulty, this incurs increase in cost because such a heating element is expensive.
Meanwhile, in order to make longer the lifetime (to improve the durability) of the heating element by reducing a load applied thereto, it is effective to increase a ratio of a surface area of the heating element (element surface area) relative to a supplied power. This is because, when the heating element surface area is increased, a surface temperature of the heating element is lowered, to thereby reduce a load of the heating element. Since a so-called spiral (helical)-type heating element can be efficiently arranged in a desired space, such a design is used for load reduction. However, as shown in FIG. 14, for example, a heater or a heat processing furnace using a spiral-type heating element conventionally employs a structure in which a heating element 18 is buried in a heat insulating member 16 so as to fix therein the heating element 18. Thus, an object to be heated in a reactor core is heated via the heat insulating member 16, so that it is difficult to quickly increase a temperature of the object. It is also difficult to quickly decrease the temperature of the object, because the heating element 18 is cooled via the heat insulating member 16, in addition to an effect of increase in heat capacity caused by the heat insulating member 16. Moreover, since there is no clearance for allowing expansion of the heating element 18, the heating element 18 itself is stressed when it is expanded. Thus, durability of the heating element may be not sufficient.
There is known a heating element that is manufactured by forming a strip-shaped heating resistance member into a waveform (Patent Document 2). Similarly to the spiral-type heating element, although this type of heating element can have an increased surface area, but has the same disadvantage in terms of installation thereof in a cylindrical insulating member.